1. Field of the Invention
The present invention is related to optical position measuring, particularly using interferometric techniques.
2. Background Art
Advancement in the accuracy of optical position measuring systems has increased dramatically over the last few years as tolerances for determining positioning have become tighter. This is especially true in a lithographic environment. As feature sizes have become dramatically smaller, very accurate alignment systems are necessary to ensure proper overlay of patterns on a substrate, wafer, flat panel display, etc. The alignment systems are used to measure current pattern position in order to accurately position the substrate and/or substrate stage for subsequent exposure of patterns. This is because even small alignment errors in pattern-to-pattern exposure can make devices inoperable.
One method of measuring position that achieves very high accuracy is interferometry using coherent light. Interferometry is based on combining two separate but coherent light beams to form an interference pattern or signal. A measuring beam interacts with a target being measured, such as a diffraction grating. Interaction with the target generates two measurement beams, which have a phase relationship that depends on the position of the target. The two measurement beams are combined to form an interference pattern or signal, which is detected and analyzed. Thus, using interferometric techniques, various relationships between the two beams, such as intensity difference and/or phase difference, for example, can be used to determine a position of a target, or portions thereof, being measured.
Coherent light sources (e.g., lasers, or the like) are used in interferometry. When using coherent light, one problem that can affect measurement accuracy is coherent interference caused by ghost or spurious reflections from various surfaces interfering with the measuring beam. As seen in FIGS. 1 and 2, these reflections can be from optical elements used to direct light to and from the target (FIG. 2).
As seen in FIGS. 1 and 2, an illumination beam 100 is directed to target 104, which may be on a wafer 102. A desired portion of a measurement beam has a positive first order component 106 (+1st order) and a negative first order component 108 (−1st order) after interaction with the target 104. It is to be appreciated that higher order components, such as 3rd, 5th, 7th, etc., may also be desired and measured. However, spurious or ghost reflections 110 and 112 (FIG. 1) or 200 (FIG. 2) are also generated. Spurious reflection 110 has an initial direction from target 104 towards optical element 114, and then reflects from optical element 114 back towards target 104 to form spurious reflection 112. For example, spurious reflections 110 and 112 are generated from the +1st order measurement beam at optical element 114, while spurious reflection 200 is generated from the illuminating beam 100. The spurious or ghost reflection 112 interferes with the minus 1st order measurement beam 108, from target plane all the way to the detection plane, including for example at a region 116 circled in FIG. 1. Similarly, spurious reflection 200 interferes with positive 1st order measurement beam 106 from the target plane all the way to the detection plane, including at a region 202 in FIG. 2.
FIG. 3 shows a phase comparison of signal 108/112 or 106/200. A phase difference between a similar point of 108/112 or 106/200 is shown as ΔZ.
FIG. 4 shows an interference pattern generated when the spurious signal 112 or 200 interferes with the measuring beam 108 or 106, respectively.
Using phase modulators to phase modulate a coherent light signal can substantially reduce or eliminate contributions to the interferometric measurement from the spurious reflections. However, phase modulators can be costly, in both money and space on the sensor, and can be complex to implement as tolerance levels continue to increase.
Therefore, what is needed is a system and method that allow for a substantial reduction and/or elimination of coherence with spurious or ghost reflections in relation to the measurement beams in optical position measuring, particularly interferometry, that are less complex and costly than conventional systems and methods.